The chemical behaviours of several phenolic compounds are compared in the presence of excited nitrite or nitrate ions. The observed reactions can be separated into three classes : oxidation, nitrosation and nitration. With monohydroxylated phenols or resorcinol in the presence of excited nitrate ions, oxidation is mainly hydroxylation, located in ortho and para positions with respect to phenol function. Hydroxylation is the only reaction initially occurring with nitro- and chlorophenols. Hydroquinone is oxidized into benzoquinone. This transformation is attributed to a reaction of hydroxyl radicals formed by photolysis of nitrite and nitrate ions. Nitrosation is the main pathway with resorcinol in the presence of nitrite ions but it also occurs with phenol and by exciting nitrate ions. It is always located in para position, but with resorcinol, 2,4-dinitrosoresorcinol can also be formed in a second step. Nitration, i.e. formation of nitrophenol; occurs by exciting nitrate ions in the presence of phenol or dihydroxybenzenes. It also occurs with nitrite ions in the presence of hydroquinone or pyrocatechol. Both nitrosation and nitration are attributed to reactions of nitrogen oxides NO·2, N2O3 or N2O4. They are unfavoured by the presence of an electro-attracting substituent such as Cl or NO2 on the ring.
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